A great many methods and substances exist to fight fires. When dealing with fires aboard aircraft, buses, trains, etc., large amounts of water are generally not available and portable containers of fire suppressants are carried. These usually consist of CO.sub.2 pressurized, dry chemicals or Halon compounds which vary in effectiveness depending upon the type of fire. For example, dry chemicals are very effective against fires where the ignition source is electrical, while the use of water may cause more problems than it solves because of the possibility of additional electrical shorts.
The instant invention is primarily concerned with extinguishers using the combination of water/foaming agents and Halon compounds, since water/foaming agents are much more effective than plain water and Halon compounds are very effective fire suppressants. For example, Halon compounds are used as a primary fire suppressant in aircraft jet engine nacelles and cargo compartments. Unfortunately, in excessive concentrations, they are poisonous to human beings. Thus, in confined areas, such as in aircraft passenger compartments, the trick is to accurately meter the Halon compounds to ensure that the Halon is thoroughly mixed with the foam to permit effective fire control while at the same time keeping the toxicity level within limits.
Initially, it will be useful to review some of the more pertinent water/foaming agents and inert gas/foaming agent extinguishers. In the latter case, German Pat. No. DE 2747588, Fire Extinguisher, by B. Gerhard is of interest. When manually activated, released nitrogen gas forces the foaming agent out of a tank to a discharge nozzle. Simultaneously, a spike, driven by nitrogen gas pressure penetrates a CO.sub.2 cartridge, releasing the gas therein. The internal mixing of the nitrogen, CO.sub.2 gas, and the foaming agent results in what is believed to be a foam of improved fire suppressing capacity. One of the problems with such a system is that the spike which penetrates the CO.sub.2 cartridge remains in place and thereby obstructs the CO.sub.2 outflow area. Also, it is not readily apparent why the combination of inert gases is any better than a single inert gas.
Another patent of interest is U.S. Pat. No. 4,106,566, Process for the Utilization of Low and Medium Expanded Foam for the Extinction of Fires from Liquefied Products, by G. Dion-Biro. Here, a medium foam expansion generator is coupled to three low expansion foam generators such that the discharge flow of the low-ratio generators will interact with the medium generator discharge flow at some distance downstream of their discharge nozzles. At this point, the medium-ratio foam is to be drawn by the low-ratio foam into an extended throw-range profile. Furthermore, the high water content of the low-ratio foam is to be compensated for by the low water content of the medium-ratio foam.
This concept raises technical questions relative to the dynamic interaction of flow streams of significantly different density, velocity, and kinetic energy. It appears likely that a sizeable portion of the relatively fragile medium-ratio foam will be destroyed in this process.
Of further interest is U.S. Pat. No. 3,979,326, Dry Foam Producing Apparatus, by J. Chatterton which is intended to produce large quantities of what is identified as "dry foam". This is to be accomplished by use of an autoclave-like pressure vessel supplied with compressed air and a detergent/water solution by auxiliary subsystems. A quantity of the detergent/water solution is provided in the basin portion of the vessel. Located above the basin are two perforated conical plates which are joined to the inner surface of the vessel such that their apex is pointed downward toward the basin. The apex of the plates and the basin are connected by a drain tube. By injecting compressed air into the detergent/water solution, a large foam flow is generated. This foam, due to its volumetric expansion, is forced through the perforated conical plates where a significant quantity of liquid is removed from each foam bubble. The "removed" liquid is returned to the basin. After this process, the foam passes through an exit aperture where, through filtering, more moisture is extracted. Furthermore, chemicals may be applied to the external surfaces of each bubble. Following this process, the so-called " dry foam" is ready for application.
This process causes destruction of the large bubbles by forcing them through the small holes in the plates. The perforated cones are a significant flow area reduction with a corresponding pressure drop. Actually, the destruction of the large bubble results in liberation of the foaming fluid, a part of which is converted into small bubbles while the remainder drains back to the basin.
The above three patents are of interests in that they disclose state-of-the-art methods for generating foam-type fire suppressant. None, of course, deal with the use of a third fire suppressant chemical such as Halon.
There are numerous systems making use of Halon compounds. As previously mentioned, pure pressurized Halon is ideally suitable for use in jet engine nacelles and the like where there obviously are no people present. Although not entirely clear, it is believed that the Halon interfers directly with the combustion process making it ideally suited in such applications. An example of the use of Halon compounds in fire extinguishers can be found in U.S. Pat. No. 4,069,872, Method of and Device for Extinguishing Burning Gases, by H. Lassen. This device is intended to suppress a fire in the bleed-off vent of a low temperature liquid gas tank. To this end, a coaxial cone arrangement, of which the lower part forms a divergent area and the outer upper section forms a convergent area, is utilized. The cone assembly also contains Halon discharge nozzles and provisions for supplemental combustion air. In the event of a fire, the Halon is discharged between the vent exit and the combustion area. Here, as in the case of the jet engine nacelle, there are no people present and, thus, very little control over the amount of Halon released is necessary. In fact, in such a situation, the more, the better.
Also of interest is U.S. Pat. No. 4,390,069, Trifluorobromomethane Foam Fire Fighting System by G. R. Rose, Jr. In this device, it is intended that the fire suppressant characteristics of a low expansion ratio water/foaming agent solution be improved by the use of a Halon compound. This is achieved by the injection of Halon 1301 in its liquid phase into a low expansion ratio foam water/agent solution which is utilized in a low expansion ratio foam generator. The Halon storing and supply system consists of a pressure vessel wherein the Halon is retained at its saturated vapor pressure in its liquid phase. Furthermore, a shut-off valve, check valve, and associated tubing are provided through which the Halon is conveyed to the point of injection. Liquid phase Halon is injected into: (1) a flowing confined stream of low expansion ratio foam solution, (2) into a low expansion ratio foam solution proportioning and pressurization pump, and (3) into the air induction ports of a low expansion ratio foam generator. All of these foam generators are intended to utilize solutions containing 1.5 to 6% of foaming agents. They are limited to a maximum expansion ratio of 1 to 15.
There are some technical questions raised by such a system. The saturated vapor pressure of Halon 1301 at 70.degree. F. is 200 psig. This is the expulsion force in the illustrated system. (It actually is 214 psig at the quoted temperature.) What is not disclosed is that this pressure varies significantly as a function of temperature. For instance, at 30.degree. F. the pressure is 103.9 psig and at 100.degree. F. it is 300.6 psig.
In addition to significant pressure variations, the viscosity of the liquid also changes. For instance, the viscosity of liquid Halon at 30.degree. F. is 0.240 centipoise, at 100.degree. F. the viscosity is 0.141 centipoise. A proportional change in the fluid density also occurs as a function of temperature and pressure.
These variations of saturated vapor pressure, viscosity, and density of liquid Halon 1301 are irrelevant if the Halon is utilized in a supercharged high rate discharge system in which the total content of the pressure vessel is expelled in a fraction of a second. The same applies to nonsupercharged, small, hand-held fire extinguishers which are utilized for point protection. However, in a system where a finite quantity of Halon is to be metered over an extended time period, these variations become critical. In fact, without controlled compensation for variations of temperature and pressure, it is impossible to meter a finite quantity of Halon 1301 in its liquid state. Therefore, the Halon concentration level is not effectively controlled by fixed geometry metering orifices.
The assumption that Halon 1301, when injected in its liquid phase into a flowing pressurized confined stream of low expansion ratio foam solution, will, upon discharge of the solution, result in foam bubbles which are stable, is questionable for the following reasons:
1. On high capacity, low expansion ratio foam generators, the discharge nozzle pressures are high (above 100 psig). This means that any expansion of liquid-phase Halon within the pressurized flowing stream of foam solution is limited by the stream pressure. Consequently, the vaporization (expansion) of the Halon will occur abruptly when the stream leaves the discharge nozzle.
2. Since the type of foaming agent which is used with the above identified foam generator is limited to a maximum expansion ratio of approximately 1 to 15 (one of the limits is the membrane strength of the bubbles), full expansion of the Halon to prevailing ambient pressure, most likely, will result in rupture of the membrane and, consequently, a loss of Halon.
3. If Halon 1301 (although not water soluble) is injected into water in its liquid phase, hydrate formation (solid crystals) may occur under certain pressures and temperature conditions. While not mentioned in the patent, medium-to-high expansion ratio generators (50-1000 to 1), which are significantly different from low expansion ratio foam generating devices, cannot process liquid-phase Halon in the encapsulation sequence.
In conclusion, the system does not maintain the Halon storage vessel at constant pressure, does not provide compensation for variations in liquid temperature or viscosity changes, does not compensate for transfer line losses, nor does it provide for the total expansion of the Halon compound to prevailing ambient pressure levels. Of most importance, it can not control the Halon concentration level in the bubbles to a level of 5% or lower. Finally, it can not, if required by the fire characteristics, increase the Halon concentration level in the bubbles up to 15% for rapid fire knock down.
Other patents of interest are U.S. Pat. No. 2,819,764, Fire Extinguishing Apparatus, by C. Anthony, Jr., U.S. Pat. No. 3,384,182, Method and Apparatus for Extinguishing Fires Utilizing a Single Aqueous Solution of a Salt and a Foaming Agent, by G. Rotvand, U.S. Pat. No. 3,804,759, Aerosol Fire Extinguisher and Method, by J. R. Becker et al., U.S. Pat. No. 3,998,274, Fire Extinguisher Head Assembly, by J. P. Liautaud, U.S. Pat. No. 4,088,194, Combination Gauge Shield and Lock Pin for a Fire Extinguisher, by H. D. Hard, and U.S. Pat. No. 4,164,960, Apparatus for Mixing Fluids, by C. W. Howard.
Therefore, it is a primary object of the subject invention to provide a fire suppression system using water/foaming agent-Halon compounds.
A further object of the subject invention is to provide a fire suppression system that is capable of actually adjusting the amount of Halon that is delivered to the fire by the water/foaming agent-Halon mixture.
It is still another object of the subject invention to provide a fire extinguisher system that utilizes hermetically sealed containers for the Halon and the inert pressurizing gas which can be used to provide a constant expulsion force for the Halon.